Optical glare is familiar when objects are viewed through partially transparent media such as windows, spectacle lenses, windshields, goggles, video monitors, and the like. Glare reduces one's ability to resolve detail in the field of view and can be distracting or even disabling. Referring to FIG. 1, a primary image is formed from a series of light rays that propagate through a partially transparent device, the pupil of an observer, and then onto their retina. If the light rays strike the device at normal incidence there is no refraction, though a proportion of the incident light is reflected at each air-device interface according to Fresnel's equations. For example, if the device is a flat plate of glass (nglass˜1.5) and nair˜1.0 then 4% is reflected from the first interface and 3.84% (=0.04*0.96) is reflected at the second, leading to transmission of 92.16% of the incident ray. Although the intensity of the ray is reduced by almost 8%, there is no change in the trajectory of its propagation through the optical elements of the eye, so that the image strikes the retina with the same spatial distribution as though the partially transparent device wasn't present. There is no glare.
This situation changes dramatically if the light is incident at any angle other than ninety degrees. Referring to FIG. 1, an illuminant (101) emits light rays (102) that strike a device (103) whose refractive index differs from the surrounding medium. Some of these rays (104) propagate in the direction of the partially transparent medium (105). Other of these rays (106) are reflected in the direction of an observer (107) who perceives a reflection of the object. Some rays (108) are refracted and propagate to an observer (109) who sees a transmitted image.
An established method of reducing or eliminating the reflected and transmitted glare images treats the surfaces of the partially transparent medium to reduce or eliminate reflections. A problem with these anti-reflection coatings is that their efficacy varies with the incident angle, polarization, and wavelength of light. Another problem with anti-reflection coatings is that they are mechanically fragile or brittle; when they crack or delaminate their contribution to glare reduction is eliminated. Yet another problem with these coatings is that they require expensive capital equipment for precisely controlled deposition of very thin films. Another problem with anti-reflection coatings is that deposition over large substrates such as automotive windshields or architectural glass is impractical. For these and other reasons an improved method for reducing the intensity of glare images, whether transmitted or reflected, is desirable.
Another problem with prior art is that it fails to account for the psychophysics of human perception. While it is widely known that the perception of light intensity varies with wavelength and average field illuminance, described as photopic (bright light), scotopic (night vision) and intermediate (mesopic) sensitivities, the perception of glare is different, as set forth for example in Fekete et al., Ophthalmic and Physiological Optics, 2010, 30, 182-187.
Yet another problem with prior art is that the wide dynamic range and nonlinearity of the human visual system are not explicitly or adequately incorporated into the design of glare reducing methods and devices.